Fuel cell stack

ABSTRACT

A casing of a fuel cell stack has stack deformation prevention structure for limiting the change of an interval between end plates on the lower side in a direction of gravity, due to swelling of the lower side of the stack body in the direction of gravity. The stack deformation prevention structure is configured such that elastic modulus of a side plate provided on a lower side of the stack body in the direction of gravity is higher than elastic modulus of a side plate provided on an upper side of the stack body in the direction of gravity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel cell stack which includes astack body formed by stacking a plurality of unit cells in a horizontaldirection, and a pair of end plates sandwiching the stack body. Each ofthe unit cells includes an electrolyte electrode assembly and separatorssandwiching the electrolyte electrode assembly. The electrolyteelectrode assembly includes a pair of electrodes and an electrolyteinterposed between the electrodes.

2. Description of the Related Art

For example, a solid polymer electrolyte fuel cell employs anelectrolyte membrane (electrolyte) comprising a polymer ion exchangemembrane. The electrolyte membrane is interposed between an anode and acathode to form a membrane electrode assembly. The membrane electrodeassembly is sandwiched between separators to form a fuel cell. In use,normally, a predetermined number of (e.g., several tens to severalhundreds of) fuel cells are stacked together to form a fuel cell stackto obtain the desired electrical energy.

At the time of power generation in the fuel cell, by electrochemicalreactions of hydrogen and oxygen, water is produced. Therefore, thepower generation performance tends to be changed easily depending on theinternal state of the produced water. Therefore, the state of theproduced water needs to be managed suitably.

In this regard, for example, a fuel cell apparatus as disclosed inJapanese Laid-Open Patent Publication No. 2001-319673 is known. In theconventional technique, as shown in FIG. 16, a fuel cell stack 3 and acompression stress regulator mechanism 4 are provided. Hydrogen from ahydrogen supply apparatus 1 and oxygen from an oxygen supply apparatus 2are used as fuels for power generation in the fuel cell stack 3. Thecompression stress regulator mechanism 4 regulates compression stressapplied to the fuel cell stack 3.

The compression stress regulator mechanism 4 includes a surface pressureapplying member 5, a spherical body 6, a screw 7, and a motor 8. Thesurface pressure applying member 5 is attached to an end of the fuelcell stack 3. The surface pressure applying member 5 applies a surfacepressure to the fuel cell stack 3. The spherical body 6 applies an axialforce uniformly to the surface pressure applying member 5. The screw 7applies the axial force to the spherical body 6. The motor 8 rotates thescrew 7.

According to the disclosure, by operation of the compression stressregulator mechanism 4, compression stress is regulated to adjust thespace for movement of water in the fuel cell stack 3 to achieve thedesired humidification state in the fuel cell stack 3.

In the fuel cell stack 3, swelling of the electrolyte membrane occurs bythe water produced in the power generation. In particular, swelledportion becomes large, in particular, on the lower side in the directionof gravity. Thus, difference in swelling occurs in the electrolytemembrane along the direction of gravity.

However, in the conventional technique, the spherical body 6 pressessubstantially the center of the surface pressure applying member 5attached to the end of the fuel cell stack 3, and the swellingdifference in the direction of gravity, in the electrolyte membranecannot be eliminated. Thus, for example, when the fuel cell stack 3 isplaced in a casing (box), the load is applied non-uniformly to the fuelcell stack 3 due to the difference in swelling. As a result, the casingis deformed undesirably.

SUMMARY OF THE INVENTION

The present invention has been made to solve the problem of this type,and an object of the present invention is to provide a fuel cell stackin which stack deformation due to swelling difference in a direction ofgravity of the electrolyte is suppressed suitably.

The present invention relates to a fuel cell stack which comprises astack body formed by stacking a plurality of unit cells in a horizontaldirection. A pair of end plates sandwiches the stack body. Each of theunit cells includes an electrolyte electrode assembly and separatorssandwiching the electrolyte electrode assembly. The electrolyteelectrode assembly includes a pair of electrodes and an electrolyteinterposed between the electrodes.

The fuel cell stack has stack deformation prevention structure forlimiting a change in an interval between the end plates on a lower sideof the stack body in the direction of gravity to be not greater than achange in an interval between the end plates on an upper side of thestack body in the direction of gravity, due to swelling on the lowerside of the stack body in the direction of gravity.

The lower side of the stack body in the direction of gravity hereinmeans the lower side relative to the center of the stack body in thedirection of gravity. The upper side of the stack body in the directionof gravity herein means the upper side relative to the center of thestack body in the direction of gravity.

In the present invention, in the presence of the stack deformationprevention structure, the change in the interval between the end plateson the lower side of the stack body in the direction of gravity islimited to be not greater than the change in the interval between theend plates on the upper side of the stack body in the direction ofgravity, due to the swelling on the lower side of the stack body in thedirection of gravity. Thus, stack deformation due to swelling of theelectrolyte is suppressed suitably.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which preferredembodiments of the present invention are shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial exploded perspective view schematically showing afuel cell stack according to a first embodiment of the presentinvention;

FIG. 2 is a partial cross sectional side view showing the fuel cellstack;

FIG. 3 is an exploded perspective view showing a unit cell of the fuelcell stack;

FIG. 4 is a perspective view showing the fuel cell stack;

FIG. 5 is a partial exploded perspective view schematically showing afuel cell stack according to a second embodiment of the presentinvention;

FIG. 6 is partial cross sectional side view showing a fuel cell stackaccording to a third embodiment of the present invention;

FIG. 7 is a side view showing a unit cell of a fuel cell stack accordingto a fourth embodiment of the present invention;

FIG. 8 is a side view showing a unit cell of a fuel cell stack accordingto a fifth embodiment of the present invention;

FIG. 9 is a partial cross sectional view showing a fuel cell stackaccording to a sixth embodiment of the present invention;

FIG. 10 is an exploded perspective view showing a unit cell of a fuelcell stack according to a seventh embodiment of the present invention;

FIG. 11 is a cross sectional view showing the unit cell, taken along aline XI-XI in FIG. 10;

FIG. 12 is a cross sectional view showing the unit cell, taken along aline XII-XII in FIG. 10;

FIG. 13 is an exploded perspective view showing a unit cell of a fuelcell stack according to an eighth embodiment of the present invention;

FIG. 14 is a cross sectional view showing the unit cell, taken along aline XIV-XIV in FIG. 13;

FIG. 15 is a cross sectional view showing the unit cell, taken along aline XV-XV in FIG. 13; and

FIG. 16 is a view showing a conventional fuel cell apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIGS. 1 and 2, a fuel cell stack 10 according to a firstembodiment of the present invention includes a stack body 14 formed bystacking a plurality of unit cells 12 in a horizontal directionindicated by an arrow A. At one end of the stack body 14 in a stackingdirection indicated by the arrow A, a terminal plate 16 a is provided.An insulating plate 18 a is provided outside the terminal plate 16 a,and an end plate 20 a is provided outside the insulating plate(insulator) 18 a. At the other end of the stack body 14 in the stackingdirection, a terminal plate 16 b is provided. An insulating plate 18 b(insulator) is provided outside the terminal plate 16 b, and an endplate 20 b is provided outside the insulating plate 18 b. An insulatingspacer member may be used as the insulating plate 18 b. The fuel cellstack 10 is placed in a box-shaped casing 24 including the rectangularand vertically elongate end plates 20 a, 20 b.

As shown in FIGS. 2 and 3, each of the unit cells 12 is formed bysandwiching a membrane electrode assembly (electrolyte electrodeassembly) 30 between a first metal separator 32, and a second metalseparator 34. The first metal separator 32 and the second metalseparator 34 are thin corrugated metal plates. Each of the membraneelectrode assemblies 30 and the first and second metal separators 32, 34has a rectangular and vertically elongate shape. Instead of the firstand second metal separators 32, 34, for example, carbon separators maybe used.

At one end (upper end) of the unit cell 12 in a longitudinal directionindicated by an arrow C in FIG. 3, an oxygen-containing gas supplypassage 36 a for supplying an oxygen-containing gas and a fuel gassupply passage 38 a for supplying a fuel gas such as ahydrogen-containing gas are provided. The oxygen-containing gas supplypassage 36 a and the fuel gas supply passage 38 a extend through theunit cell 12 in the direction indicated by the arrow A.

At the other end (lower end) of the unit cell 12 in the longitudinaldirection, a fuel gas discharge passage 38 b for discharging the fuelgas and an oxygen-containing gas discharge passage 36 b for dischargingthe oxygen-containing gas are provided. The fuel gas discharge passage38 b and the oxygen-containing gas discharge passage 36 b extend throughthe unit cell 12 in the direction indicated by the arrow A.

At one end of the unit cell 12 in a lateral direction indicated by anarrow B, a coolant supply passage 40 a for supplying a coolant isprovided. At the other end of the unit cell 12 in the lateral direction,a coolant discharge passage 40 b for discharging the coolant isprovided.

The membrane electrode assembly 30 includes an anode 44, a cathode 46,and a solid polymer electrolyte membrane 42 interposed between the anode44 and the cathode 46. The solid polymer electrolyte membrane 42 isformed by impregnating a thin membrane of perfluorosulfonic acid withwater, for example.

Each of the anode 44 and the cathode 46 has a gas diffusion layer (notshown) such as a carbon paper, and an electrode catalyst layer (notshown) of platinum alloy supported on porous carbon particles. Thecarbon particles are deposited uniformly on the surface of the gasdiffusion layer. The electrode catalyst layer of the anode 44 and theelectrode catalyst layer of the cathode 46 are formed on both surfacesof the solid polymer electrolyte membrane 42, respectively.

The first metal separator 32 has a fuel gas flow field 48 on its surface32 a facing the membrane electrode assembly 30. The fuel gas flow field48 extends in the direction indicated by the arrow C, and the fuel gasflow field 48 is connected between the fuel gas supply passage 38 a andthe fuel gas discharge passage 38 b. A coolant flow field 50 is formedon a surface 32 b of the first metal separator 32. The coolant flowfield 50 extends in the direction indicated by the arrow B, and thecoolant flow field 50 is connected between the coolant supply passage 40a and the coolant discharge passage 40 b.

The second metal separator 34 has an oxygen-containing gas flow field 52on its surface 34 a facing the membrane electrode assembly 30. Theoxygen-containing gas flow field 52 extends in the direction indicatedby the arrow C, and the oxygen-containing gas flow field 52 is connectedbetween the oxygen-containing gas supply passage 36 a and theoxygen-containing gas discharge passage 36 b. The coolant flow field 50is formed on a surface 34 b of the second metal separator 34. That is,the coolant flow field 50 is formed by overlapping the surface 34 b ofthe second metal separator 34 and the surface 32 b of the first metalseparator 32.

A first seal member 54 is formed integrally on the surfaces 32 a, 32 bof the first metal separator 32, around the outer end of the first metalseparator 32. A second seal member 56 is formed integrally on thesurfaces 34 a, 34 b of the second metal separator 34, around the outerend of the second metal separator 34.

As shown in FIG. 2, a seal 57 is interposed between the first and thesecond seal member 54, 56 for preventing the outer end of the solidpolymer electrolyte membrane 42 from directly contacting the casing 24.

As shown in FIG. 1, a rod shaped terminal 58 a is provided atsubstantially the center of the terminal plate 16 a, and a rod shapedterminal 58 b is provided at substantially the center of the terminalplate 16 b. The rod shaped terminals 58 a, 58 b protrude in the stackingdirection. The terminals 58 a, 58 b pass through holes 59 a, 59 b formedat the center of the end plates 20 a, 20 b in the longitudinal directionand the lateral direction, and protrude to the outside. For example, aload such as a travel motor is connected to the terminals 58 a, 58 b.

As shown in FIG. 1, the casing 24 includes the end plates 20 a, 20 b, aplurality of side plates 60 a to 60 d, angle members 62 a to 62 d, andcoupling pins 64 a, 64 b. The side plates 60 a to 60 d are provided onsides of the stack body 14. The angle members 62 a to 62 d are used forcoupling adjacent ends of the side plates 60 a to 60 d together. Thecoupling pins 64 a, 64 b are used for coupling the end plates 20 a, 20 band the side plates 60 a to 60 d. The coupling pins 64 a, 64 b havedifferent lengths.

For example, the side plates 60 a to 60 d are thin metal plates. Theside plates 60 a to 60 d and the angle members 62 a to 62 d are fixedtogether using bolts 65 to form the casing 24 (see FIG. 4).

Each of upper and lower ends of the end plate 20 a has one first hinge66 a. Each of upper and lower ends of the end plate 20 b has one firsthinge 66 b. Each of left and right ends of the end plate 20 a has twofirst hinges 66 c. Each of left and right ends of the end plate 20 b hastwo first hinges 66 d.

The side plates 60 a, 60 c are provided on opposite sides of the stackbody 14 in the direction indicated by the arrow B. Each longitudinal endof the side plate 60 a in the longitudinal direction indicated by thearrow A has three second hinges 70 a. Each longitudinal end of the sideplate 60 c in the longitudinal direction indicated by the arrow A hasthree second hinges 70 b. The side plate 60 b is provided on the upperside of the stack body 14, and the side plate 60 d is provided on thelower side of the stack body 14. Each longitudinal end of the side plate60 b has two second hinges 72 a. Each longitudinal end of the side plate60 d has two second hinges 72 b.

As shown in FIG. 4, the first hinges 66 c of the end plate 20 a, and thefirst hinges 66 d of the end plate 20 b are positioned between thesecond hinges 70 a of the side plate 60 a, and between the second hinges70 b of the side plate 60 c. The long coupling pins 64 a are insertedinto these hinges 66 c, 66 d, 70 a, 70 b.

Likewise, the second hinges 72 a of the side plate 60 b and the firsthinges 66 a, 66 b of the upper ends of the end plates 20 a, 20 b arepositioned alternately, and the second hinges 72 b of the side plate 60d and the first hinges 66 a, 66 b of the lower ends of the end plates 20a, 20 b are positioned alternately. The short coupling pins 64 b areinserted into these hinges 66 a, 66 b, 72 a, 72 b.

As shown in FIG. 1, an oxygen-containing gas inlet 76 a and a fuel gasinlet 78 a are provided in the end plate 20 a. The oxygen-containing gasinlet 76 a is connected to the oxygen-containing gas supply passage 36a, and the fuel gas inlet 78 a is connected to the fuel gas supplypassage 38 a. Further, an oxygen-containing gas outlet 76 b and a fuelgas outlet 78 b are provided in the end plate 20 a. Theoxygen-containing gas outlet 76 b is connected to the oxygen-containinggas discharge passage 36 b, and the fuel gas outlet 78 b is connected tothe fuel gas discharge passage 38 b.

A coolant inlet 80 a and a coolant outlet 80 b are provided in the endplate 20 b. The coolant inlet 80 a is connected to the coolant supplypassage 40 a, and the coolant outlet 80 b is connected to the coolantdischarge passage 40 b.

The casing 24 has stack deformation prevention structure 82 for limitingthe change in the interval between the end plates 20 a, 20 b on thelower side in a direction of gravity due to swelling on the lower sideof the stack body 14 in the direction of gravity.

The lower side of the stack body 14 in the direction of gravity hereinmeans the lower side relative to the center of the stack body 14 in thedirection of gravity. The upper side of the stack body 14 in thedirection of gravity herein means the upper side relative to the centerof the stack body 14 in the direction of gravity. By swelling of thelower side of the stack body 14 in the direction of gravity, theinterval between the end plates 20 a, 20 b changes in a direction awayfrom each other, with gradient from the upper side to the lower side inthe direction of gravity.

The change in the interval between the end plates 20 a, 20 b due toswelling of the stack body 14 depends on the total deformation amount inthe stacking direction indicated by the arrow A of the solid polymerelectrolyte membranes 42 of the membrane electrode assemblies 30 of therespective unit cells 12, when the solid polymer electrolyte membranes42 are swelled by water.

The stack deformation prevention structure 82 is configured such thatelastic modulus in the stacking direction of the side plate 60 dprovided on the lower side of the stack body 14 in the direction ofgravity becomes higher than elastic modulus in the stacking direction ofthe side plate 60 b provided on the upper side in the direction ofgravity. Specifically, a plurality of thick portions (or separate platemembers) 84 extending in the direction indicated by the arrow A areprovided on the bottom side of the side plate 60 d. Alternatively, thethickness of the side plate 60 d on the lower side may be larger thanthe thickness of the side plate 60 b on the upper side.

Next, operation of the fuel cell stack 10 will be described below.

As shown in FIG. 4, an oxygen-containing gas is supplied to theoxygen-containing gas inlet 76 a of the end plate 20 a, and a fuel gassuch as a hydrogen-containing gas is supplied to the fuel gas inlet 78a. Further, a coolant such as pure water or ethylene glycol is suppliedto the coolant inlet 80 a of the end plate 20 b.

Thus, in the stack body 14 formed by stacking the unit cells 12 in thedirection indicated by the arrow A, the oxygen-containing gas, the fuelgas, and the coolant are supplied to the oxygen-containing gas supplypassage 36 a, the fuel gas supply passage 38 a and the coolant supplypassage 40 a in the direction indicated by the arrow A.

As shown in FIG. 3, the oxygen-containing gas is supplied from theoxygen-containing gas supply passage 36 a to the oxygen-containing gasflow field 52 of the second metal separator 34, and flows along thecathode 46 of the membrane electrode assembly 30. The fuel gas issupplied from the fuel gas supply passage 38 a to the fuel gas flowfield 48 of the first metal separator 32, and flows along the anode 44of the membrane electrode assembly 30.

Thus, in each of the membrane electrode assemblies 30, theoxygen-containing gas supplied to the cathode 46, and the fuel gassupplied to the anode 44 are partially consumed in the electrochemicalreactions at catalyst layers of the cathode 46 and the anode 44 forgenerating electricity.

Then, the oxygen-containing gas partially consumed at the cathode 46flows along the oxygen-containing gas discharge passage 36 b, and isdischarged to the outside through the oxygen-containing gas outlet 76 bat the end plate 20 b (see FIG. 4). Likewise, the fuel gas partiallyconsumed at the anode 44 flows through the fuel gas discharge passage 38b, and is discharged to the outside through the fuel gas outlet 78 b atthe end plate 20 a.

Further, the coolant flows into the coolant flow field 50 between thefirst and second metal separators 32, 34 from the coolant supply passage40 a, and flows in the direction indicated by the arrow B. After thecoolant cools the membrane electrode assembly 30, the coolant movesthrough the coolant discharge passage 40 b, and the coolant isdischarged through the coolant outlet 80 b at the end plate 20 b (seeFIG. 1).

In the embodiment, as described above, when power generation isperformed in the fuel cell stack 10, in the membrane electrode assembly30 of each unit cell 12, the solid polymer electrolyte membrane 42 isswelled by water produced in the power generation. At this time, sincethe produced water moves in the direction of gravity, the lower side ofthe solid polymer electrolyte membrane 42 in the direction of gravity isswelled significantly. In particular, in the case where the membraneelectrode assembly 30 has a longitudinally elongated shape, thedifference in swelling in the direction of gravity becomes significantlylarge.

Thus, in each of the unit cells 12, the thickness on the lower side inthe direction of gravity (dimension in the direction indicated by thearrow A) becomes significantly larger than the thickness on the upperside in the direction of gravity. Therefore, a large dimensionaldifference in the stacking direction tends to occur, between the lowerside and the upper side in a vertical direction in the stack body 14 asa whole.

In the first embodiment, the casing 24 has the stack deformationprevention structure 82. The stack deformation prevention structure 82is configured such that the elastic modulus of the side plate 60 d asthe bottom plate is higher than the elastic modulus of the side plate 60b of the top plate. Therefore, even if a large stress is applied to thelower side of the stack body 14 in the direction of gravity incomparison with the upper side of the stack body 14 in the direction ofgravity, due to the difference of swelling in each of solid polymerelectrolyte membranes 42, the stress can be supported by the elasticmodulus of the side plate 60 d.

Thus, with the simple structure, the change in the interval between theend plates 20 a, 20 b is limited suitably, and damages, degradation andthe like due to deformation of the casing 24 are prevented suitably.

FIG. 5 is a partial exploded perspective view showing a fuel cell stack90 according to a second embodiment of the present invention. Theconstituent elements that are identical to those of the fuel cell stack10 according to the first embodiment are labeled with the same referencenumeral, and description thereof will be omitted. Further, also in thirdto eighth embodiments as described later, the constituent elements thatare identical to those of the fuel cell stack 10 according to the firstembodiment are labeled with the same reference numeral, and descriptionthereof will be omitted.

In the fuel cell stack 90, the side plates 60 a, 60 c of the casing 24have stack deformation prevention structure 92. Each of the side pates60 a, 60 c has two plate members 94 a, 94 b. The stack deformationprevention structure 92 is configured such that the thickness of theplate member 94 a is larger than the thickness of the plate member 94 b.Therefore, even if a large stress is applied to the lower side of thestack body 14 in the direction of gravity in comparison with the upperside of the stack body 14 in the direction of gravity, due to thedifference of swelling in each of the solid polymer electrolytemembranes 42, the stress can be supported by the elastic modulus of theplate member 94 a.

Thus, in the second embodiment, the same advantages as in the case ofthe first embodiment are obtained. In the second embodiment, each of theside plates 60 a, 60 c includes the two plate members 94 a, 94 b.Alternatively, each of the side plates 60 a, 60 c may comprise only asingle plate, and the thickness of the plate may be increasedcontinuously or stepwise from the upper side of the stack body 14 in thedirection of gravity to the lower side of the stack body 14 in thedirection of gravity.

FIG. 6 is a partial cross sectional view showing a fuel cell stack 100according to a third embodiment of the present invention.

In the fuel cell stack 100, each of the unit cells 12 has stackdeformation prevention structure 102. The stack deformation preventionstructure 102 is configured such that elastic modulus of ends 54 a, 56 aon the upper side of the first and second seal members 54, 56 in thedirection of gravity is higher than elastic modulus of ends 54 b, 56 bon the lower side of the first and second seal members 54, 56 in thedirection of gravity. Specifically, cross sectional areas of the firstand second seal members 54, 56 or materials of the first and second sealmembers 54, 56 are changed for changing the elastic modulus.

In the third embodiment, elastic modulus of the ends 54 a, 56 a on theupper side of the first and second seal members 54, 56 in the directionof gravity is higher than elastic modulus of the ends 54 b, 56 b on thelower side of the first and second seal members 54, 56 in the directionof gravity. Therefore, the load supported by the ends 54 a, 56 a of thefirst and second seal members 54, 56 is larger than the load supportedby the ends 54 b, 56 b of the first and second seal members 54, 56.

Therefore, even if swelling of the lower side of each unit cell 12 inthe direction of gravity becomes large, the interval between the firstand second metal separators 32, 34 on the lower side in the direction ofgravity does not become large, because the interval between the firstand second metal separators 32, 34 on the upper side in the direction ofgravity is not narrowed in the presence of the ends 54 a, 56 a havinghigh elastic modulus. Accordingly, overall deformation of the fuel cellstack 100 in the stacking direction is prevented effectively. Thus, thesame advantages as in the case of the first embodiment are obtained.

FIG. 7 is a side view showing a unit cell 12 of a fuel cell stack 110according to a fourth embodiment of the present invention.

In the fuel cell stack 110, each of the unit cells 12 has stackdeformation prevention structure 112. The stack deformation preventionstructure 112 is configured such that the thickness (t1) on the lowerside of the first and second metal separators 32, 34 in the direction ofgravity is smaller than the thickness (t2) on the upper side of thefirst and second metal separators 32, 34 in the direction of gravity(t1<t2).

In the fourth embodiment, in each of unit cells 12, deformation in thestacking direction occurs easily on the lower side in the direction ofgravity, in comparison with the upper side in the direction of gravity.It is because the thickness (t1) on the lower side of the first andsecond metal separators 32, 34 is smaller than the thickness (t2) on theupper side of the first and second metal separators 32, 34. Therefore,when the lower side of the solid polymer electrolyte membrane 42 of eachunit cell 12 in the direction of gravity is swelled to a great extent incomparison with the upper side of the solid polymer electrolyte membrane42 in the direction of gravity due to power generation, the first andsecond metal separators 32, 34 on the lower side in the direction ofgravity are deformed easily in the stacking direction.

Thus, swelling on the lower side of each solid polymer electrolytemembrane 42 in the direction of gravity is absorbed easily bydeformation of the first and second metal separators 32, 34, and thedimension (interval) in the stacking direction between the end plates 20a, 20 b does not change in the fuel cell stack 110 as a whole.

FIG. 8 is a side view showing a unit cell 12 of a fuel cell stack 120according to a fifth embodiment of the present invention.

In the fuel cell stack 120, each of the unit cells 12 has stackdeformation prevention structure 122. The stack deformation preventionstructure 122 is configured such that the thickness (t3) on the lowerside of the solid polymer electrolyte membrane 42 of the membraneelectrode assembly 30 in the direction of gravity is smaller than thethickness (t4) on the upper side of the solid polymer electrolytemembrane 42 in the direction of gravity.

In the fifth embodiment, at the time of power generation in the fuelcell stack 120, the solid polymer electrolyte membrane 42 is swelled byabsorption of water produced in the power generation. The thickness (t3)on the lower side of the solid polymer electrolyte membrane 42 in thedirection of gravity, i.e., the thickness on the side where the amountof the produced water is large is smaller than the thickness (t4) on theupper side of the solid polymer electrolyte membrane 42 in the directionof gravity, i.e., the thickness on the side where the amount of theproduced water is small.

Thus, by swelling, the thickness of the solid polymer electrolytemembrane 42 becomes substantially uniform along the direction ofgravity, and it becomes possible to inhibit application of thenon-uniform load to the fuel cell stack 120.

FIG. 9 is a partial cross sectional view showing a fuel cell stack 130according to a sixth embodiment of the present invention.

The fuel cell stack 130 has stack deformation prevention structure 132.The stack deformation prevention structure 132 is configured such thattapered surfaces 134 a, 134 b are provided in each of the inner surfacesof the end plates 20 a, 20 b, and the tapered surfaces 134 a, 134 b areslanted outwardly, toward the lower side in the direction of gravity.The interval between the end plates 20 a, 20 b on the lower side in thedirection of gravity is larger than the interval between the end plates20 a, 20 b on the upper side in the direction of gravity (see distancet5).

In the sixth embodiment, in each of the unit cells 12, when the lowerside in the direction of gravity is swelled to a greater extent incomparison with the upper side in the direction of gravity, since theinterval between the end plates 20 a, 20 b on the lower side in thedirection of gravity is larger than the interval between the end plates20 a, 20 b on the upper side in the direction of gravity, the differencein swelling is absorbed in each of the unit cells 12. Thus, whenswelling occurs in each unit cell 12 on the lower side in the directionof gravity, deformation of the fuel cell stack 130 is limitedadvantageously.

In the sixth embodiment, the end plates 20 a, 20 b have the taperedsurfaces 134 a, 134 b. Alternatively, or in addition to this structure,the insulating plates 18 a, 18 b or the terminal plates 16 a, 16 b mayhave the similar tapered surfaces (not shown).

FIG. 10 is an exploded perspective view showing a unit cell 140 of afuel cell stack according to a seventh embodiment of the presentinvention.

The unit cell 140 has first and second metal separators 142, 144sandwiching the membrane electrode assembly 30. The first and secondmetal separators 142, 144 are corrugated thin plates. By corrugating thefirst metal separator 142, a fuel gas flow field 48 is formed on asurface of the first metal separator 142 facing the membrane electrodeassembly 30, and by corrugating the second metal separator 144, anoxygen-containing gas flow field 52 is formed on a surface of the secondmetal separator 144 facing the membrane electrode assembly 30.

The fuel gas flow field 48 and the oxygen-containing gas flow field 52has a cross sectional shape as shown in FIG. 11 on the upper side of thestack body 14 in the direction of gravity and a cross sectional shape asshown in FIG. 12 on the lower side of the stack body 14 in the directionof gravity. In the fuel gas flow field 48 and the oxygen-containing gasflow field 52, elastic modulus in the stacking direction, on the lowerside of the stack body 14 in the direction of gravity is smaller thanelastic modulus in the stacking direction, on the upper side of thestack body 14 in the direction of gravity. That is, the lower side ofthe stack body 14 can be deformed easily.

Thus, in the seventh embodiment, the unit cell 140 is deformed easily inthe stacking direction, on the lower side in the direction of gravity,in comparison with the upper side in the direction of gravity.Therefore, the same advantages as in the case of the fourth embodimentare obtained. For example, swelling on the lower side of the solidpolymer electrolyte membrane 42 in the direction of gravity is absorbedeasily by deformation of the first and second metal separators 142, 144.

FIG. 13 is an exploded perspective view showing a unit cell 150 of afuel cell stack according to an eighth embodiment of the presentinvention.

The unit cell 150 has first and second metal separators 152, 154sandwiching the membrane electrode assembly 30. The first and secondmetal separators 152, 154 are corrugated thin plates. By corrugating thefirst metal separator 152, a fuel gas flow field 48 is formed on asurface of the first metal separator 152 facing the membrane electrodeassembly 30, and by corrugating the second metal separator 154, anoxygen-containing gas flow field 52 is formed on a surface of the secondmetal separator 154 facing the membrane electrode assembly 30.

The fuel gas flow field 48 and the oxygen-containing gas flow field 52has a cross sectional shape as shown in FIG. 14 on the upper side of thestack body 14 in the direction of gravity and a cross sectional shape asshown in FIG. 15 on the lower side of the stack body 14 in the directionof gravity. In the fuel gas flow field 48 and the oxygen-containing gasflow field 52, elastic modulus in the stacking direction, on the lowerside of the stack body 14 in the direction of gravity is smaller thanelastic modulus in the stacking direction, on the upper side of thestack body 14 in the direction of gravity. That is, the lower side ofthe stack body 14 can be deformed easily.

Therefore, in the eighth embodiment, the same advantages as in the caseof the seventh embodiment are obtained. For example, swelling in thedirection of gravity, on the lower side of the solid polymer electrolytemembrane 42 is absorbed easily by deformation of the first and secondmetal separators 152, 154.

While the invention has been particularly shown and described withreference to preferred embodiments, it will be understood thatvariations and modifications can be effected thereto by those skilled inthe art without departing from the scope of the invention as defined bythe appended claims.

1. A fuel cell stack comprising: a stack body formed by stacking aplurality of unit cells in a horizontal direction, a pair of end platessandwiching the stack body, the unit cells each including an electrolyteelectrode assembly and separators sandwiching the electrolyte electrodeassembly, the electrolyte electrode assembly including a pair ofelectrodes and an electrolyte interposed between the electrodes; andstack deformation prevention structure for limiting a change in aninterval between the end plates on a lower side of the stack body in adirection of gravity to be not greater than a change in an intervalbetween the end plates on an upper side of the stack body in thedirection of gravity due to swelling on the lower side of the stack bodyin the direction of gravity.
 2. A fuel cell stack according to claim 1,wherein the stack deformation prevention structure is configured suchthat at least the thickness of the electrolyte or the separator on thelower side in the direction of gravity is smaller than the thickness ofthe electrolyte or the separator on the upper side in the direction ofgravity.
 3. A fuel cell stack according to claim 1, wherein the stackdeformation prevention structure is configured such that at least aninterval between the end plates or between insulators adjacent to theend plates on the lower side in the direction of gravity is larger thanan interval between the end plates or between the insulators adjacent tothe end plates on the upper side in the direction of gravity.
 4. A fuelcell stack according to claim 1, wherein the stack deformationprevention structure is configured such that elastic modulus of a sealmember of the stack body on the upper side in the direction of gravityis higher than elastic modulus of the seal member on the lower side inthe direction of gravity.
 5. A fuel cell stack according to claim 1,further comprising a casing containing the stack body, wherein thecasing includes a plurality of panel members provided around the stackbody; and the stack deformation prevention structure is configured suchthat elastic modulus of the panel member provided on the lower side inthe direction of gravity is higher than elastic modulus of the panelmember provided on the upper side in the direction of gravity.
 6. A fuelcell stack according to claim 1, further comprising a casing containingthe stack body, wherein the casing includes a plurality of panel membersprovided around the stack body; and the stack deformation preventionstructure is configured such that, in the panel members provided onlateral sides of the stack body, elastic modulus on the lower side inthe direction of gravity is higher than elastic modulus on the upperside in the direction of gravity.
 7. A fuel cell stack according toclaim 6, wherein the panel members provided on the lateral sides of thestack body each have two upper and lower plate members, and thethickness of the lower plate member is larger than the thickness of theupper plate member.
 8. A fuel cell stack according to claim 1, whereinthe stack deformation prevention structure is configured such thatelastic modulus in the stacking direction of the separators on the lowerside in the direction of gravity is smaller than elastic modulus in thestacking direction of the separators on the upper side in the directionof gravity.
 9. A fuel cell stack according to claim 1, wherein each ofthe electrolyte electrode assembly and the separators has alongitudinally elongated shape.